The present invention relates to an apparatus for extracting signals which are indicative of the level of consciousness of a patient comprising means for monitoring auditory evoked potentials (AEP) produced by the patient as a response to a repetitive acoustic click stimulus, means for extracting an AEP within a few repetitions, preferably more than 10 and less than 50 of the audio stimulus, means for using an autoregressive model with exogenous input (ARX), and means for calculating an index (AAI) indicative of anaesthetic.
Assessment of depth of anaesthesia is in general based on clinical observations of physiologic parameters such as blood pressure, heart beat rate, pupil size etc. The use of neuro-muscular blocking agents during general anaesthesia disables the clinical signs that normally indicate consciousness. A number of incidents exist where patients describe that they were fully conscious during the surgery, and in the worst case had perception of pain and cardiac arrhythmias. Hence, there is a need for a method and apparatus to assess the anaesthetic depth. A number of investigation results have already been published, where Auditory Evoked Potentials (AEP) are used to indicate the level of consciousness during general anaesthesia. The AEP is a sub-component of the EEG signal, and it is elicited by acoustic stimuli and is recorded with scalp electrodes, amplified and analysed by a computer. The AEP is an electrical, small signal embedded in noise from the ongoing EEG, and for this reason advanced signal processing is necessary to extract the AEP signals. The AEP signals are traditionally extracted by the averaging of up to 1000 repetitions of the response of the stimuli signals. This is a very time-consuming process, which takes up to several minutes to carry out, typically 2-3 minutes, which is excessive if the anaesthesiologist has to use the AEP signals as a predictor of an adequate anaesthetic dose.
From international patent application no. WO 98/10701 (PCT/GB97/02435) a control system and a method for calculating an index representation of the depth of anaesthesia are known. The method of calculating an index indication of anaesthetic depth is based on monitoring AEP produced by the patient and providing a signal corresponding to the coarseness of the monitored AEP signal. The raw AEP signal is divided into a series of sweeps or frames of a given duration, each sweep being synchronised with the repetitive audio stimuli. A number of sweeps n are recorded in sequence and are averaged to produce a time average sweep. The anaesthesia index is calculated for the time-averaged sweep. Each time a new series of sweeps is recorded, a new time-averaged sweep is determined from the most resent n sweeps, and the anaesthesia index for that time-averaged sweep is calculated. In this way the index is constantly updated.
It has been observed that when a patient loses consciousness, the amplitudes of most AEP peaks are reduced and their latencies are generally also increased. These changes occur almost simultaneously and in the same direction with all patients. Therefore, a suitable index is one, which reflects these changes.
An empirical algorithm has been developed for calculating the index, which algorithm is based upon the sum of the square roots of the difference between every two successive points in the moving time-averaged sweep. This auditory evoked potential index is given by the following equation:   AEP  =      k    ⁢                  ∑                  i          =          1                255            ⁢                                                            x              i                        -                          x                              i                +                1                                                                  where x1 to x256 are the sample points of the time-averaged frame and k is a scaling constant.
The AEP index is calculated for every filtered time-averaged sweep, and a plot of the index against time can be generated for display and on a screen. When the patient is awake, the index is typically in the range of 80 to 90, whereas during anaesthesia it is typically in the range of 35 to 40.
An article in “Methods of Information in Medicine”, 1996; 35: 256-260, with the title: “Autoregressive Modeling with Exogenous Input of Middle-Latency Auditory-Evoked Potentials to Measure Rapid Changes in Depth of Anesthesia” by E. W. Jensen, P. Lindholm and S. W. Henneberg describe a system identification method, an autoregressive model with exogenous input (ARX), to produce a sweep-by-sweep estimate of the AEP. The method was clinically evaluated in 10 patients anaesthetized with alfentanil and propofol. The time interval between propofol induction and the time when the Na—Pa amplitude was decreased to 25% of the Initial amplitude was measured. These measurements showed that ARX-estimated compared to MTA-estimated AEP was significantly faster in tracing transition from consciousness to unconsciousness during propofol induction (p<0.05).
It is the object of the invention to improve this measuring method in such a manner that a safer result is achieved considerably more rapidly, whereby the risk of treating a patient, e.g. by surgery, without full anaesthetization is reduced.
The delay is reduced to about 6 seconds by using ARX modelling.
It is a second object of the invention to make the procedure for anaesthetization more effective (time-efficient) and to reduce the staff workload.
It is a third object of the invention to produce an apparatus for continuous monitoring of the level of consciousness, which apparatus is portable and easy to install and operate.
This is achieved by using an apparatus as disclosed in claim 1.
Further advantageous characteristics are achieved by that which is disclosed in the dependent claims.
With the apparatus according to the invention the possibility is achieved of extracting the AEP with only a few repetitions, often as low as 15, which reduces the delay to approximately 6 seconds. As the AEP is a very complex signal comprising several peaks and troughs, it is desirable to map the AEP into a single number—an ARX index of easy interpretation, but containing the same information as the AEP. This is possible by following the method and by using the apparatus according to the invention. The ARX index is typically larger than 60 when the patient is awake, and decreases when the patient is anaesthetized; a loss of consciousness will typically occur when the index gets below 28.
The apparatus according to the invention can operate only with three surface electrodes, e.g. three propritary surface electrodes, the result being:                Fully updated AEP available within few seconds.        Significantly faster calculating AEP index than the traditional Moving Time Average method.        Significantly faster at tracing transition from unconsciousness and vice versa.        Consistent, accurate readings.        Possible optimised display for use in operating theatre.        Touch screen—easy to operate and clean.        Fully graphic display        
The apparatus according to the invention monitor the level of consciousness during general anaesthesia independently of the biological variation of the patients with respect to tolerance and sensitivity of the anaesthetics.
The calculations according to the invention can be performed on a computer as disclosed in claims 8 and 9.